Polymer slurry washing

ABSTRACT

A polymer slurry is passed past a filter and a wash fluid is caused to flow at a substantial angle with respect to the flow of the polymer slurry and toward the filter such as to replace at least a substantial portion of the fluid of the polymer slurry.

This is a divisional application of the copending patent applicationSer. No. 269,206, filed June 2, 1981, U.S. Pat. No. 4,421,874.

This invention relates to a process and apparatus for the washing ofpolymer slurries, more specifically the invention relates to thepurification of slurries of polymers such as polyolefins, e.g.polypropylene, poly(arylene sulfides), e.g. poly(phenylene sulfide)(PPS) and the like.

BACKGROUND OF THE INVENTION

Reactor effluents from polymerization processes comprising, for example,polypropylene can contain polymer particles, dissolved amorphouspolymer, catalyst residues, and diluent, e.g. liquid propylene, andliquid polymer. It is frequently necessary to remove one or more ofthese materials, e.g. soluble polymer from the desired product, e.g. thecrystalline polymer, in order to obtain a product having acceptablephysical properties, e.g. stiffness, tensile strength, etc. In addition,catalyst residues often must be substantially removed to minimizecorrosion of subsequent processing equipment, e.g. extruders, tominimize discoloration of the finished polymer and to reduce thequantity of stabilizer package required to protect the finished polymerfrom the deleterious effects of heat, oxygen and UV light.

Polymerization reactor effluents comprising PPS, for example, cancontain polymer particles, unreacted p-dichlorobenzene,N-methyl-2-pyrrolidone (NMP) solvent and salt (NaCl). Roughly the sameamount of salt and polymer is made in this process. It is necessary towash as much salt as possible from the polymer as well as otherash-forming by-products in order to obtain a commercially acceptableproduct. Generally, before this is done, the NMP and p-dichlorobenzeneare removed by flashing and the product is slurried in water, filtered,reslurried, etc. to remove the salt.

THE INVENTION

It is, thus, one object of this invention to provide a process for thepurification of a polymer slurry.

Another object of this invention is to provide a process wherein apolymer slurry is subjected to a washing operation.

Yet a further object of this invention is to provide a process for bothpurifying and concentrating a polymer slurry.

Still another object of this invention is to provide an apparatus forcarrying out the process of this invention.

These and other objects, advantages, details, features and embodimentsof this invention will become apparent to those skilled in the art fromthe following detailed description of the invention, the appended claimsand the drawing in which

FIG. 1 is a schematical representation of an apparatus for carrying outthe process of this invention,

FIG. 2 is a similar representation as FIG. 1 with a different controlmechanism.

In accordance with this invention a process for purifying a polymerslurry is provided which can be generically characterized as a crossextraction process. In this process the polymer slurry is moved within ahousing preferably in a settled bed plug flow state in a first directionpast a filter material allowing the fluid of the slurry to pass throughand essentially preventing substantial quantities of the polymerparticles to pass through. A wash fluid is injected into the slurry andcaused to flow at a substantial angle to the direction of movement ofthe slurry through the housing. Thereby the wash fluid replaces thefluid phase of the slurry or at least a substantial portion thereof.This process differs from regular washing or extraction operations inthat the washing fluid in accordance with this invention flows neitherconcurrently nor countercurrently with the slurry but rather flowsacross the movement of the slurry through the wash column. The movementof the slurry can in fact be achieved continuously or discontinuously.The relationship between the flow directions pointed out are the same inboth instances namely that a substantial angle exists between the flowof the wash fluid from the wash fluid inlet to the filter in the housingand the direction of the macroscopic flow of the polymer slurry.

Thus in accordance with a first embodiment of this invention a processfor the purification of a polymer slurry is provided. The polymer slurrycomprises solid polymer particles and a fluid phase containing a liquid.The fluid phase contains at least one ingredient to be at leastpartially removed from the slurry. The slurry is moved predominantly ina first direction continuously or discontinuously through a housingwhich is confined by a wall. This wall comprises at least one filterportion. In this filter portion the wall is made out of a filtermaterial permitting the fluid phase to pass through while preventing anysubstantial amount of solid polymer particles from passing through. Awash fluid is injected into said housing at a location and underpressure conditions such as to cause a flow of the wash fluid in asecond direction through the slurry. Thereby the wash fluid replaces atleast a portion of the fluid phase by the wash fluid and causes thisreplaced portion of the fluid phase to leave the housing through thefilter. Preferably most of the fluid phase of the liquid/solid mixtureis replaced by the wash fluid; the wash fluid is then employed in avolume ratio with respect to the fluid phase of 1:1 or slightly higher.The wash fluid generally contains none of said one ingredient or asubstantially less amount of the ingredient to be replaced than thepolymer slurry. The first and second flow directions form a substantialangle with each other. This angle macroscopically will be preferably inthe range of 40° to 90°; the angle between the two flow directions ishere always expressed as an angle of 90 ° or less. The purified polymerslurry is finally recovered as the product of the process from thehousing. The purified polymer slurry comprises the solid polymerparticles and a purified liquid phase containing substantially less ofthe at least one ingredient mentioned above as compared to the fluidphase of the starting polymer slurry.

It is particularly preferred to carry out the process in such a way asto establish a settled bed of the polymer slurry and in contacttherewith a settling slurry within the housing. The area between thesettled bed (or saturated bed) and the settling slurry should be keptupstream of the filter portion of said wall. The terms "downstream" or"upstream" refer to the flow of the settled bed, and thus generally tothe axial flow of the slurry through the housing. The settling of thebed of the polymer slurry is preferably achieved by liquid flow from theslurry through the bed, but may be enhanced by settling due todifferences in specific gravity between the fluid and the polymerparticles.

In a preferred embodiment of this invention the pressure conditions inthe housing are determined. Responsive to this determination thepurified polymer slurry is removed from the housing. This controloperation is best carried out by determining the pressure at least inthe portion of the column downstream of the filter portion.

Another advantageous possibility to control the process of thisinvention resides in the step of detecting and sensing the location ofthe area between the settled bed of the polymer slurry and the settlingslurry. The withdrawal of the settled bed is preferably controlledresponsive to the determination of this area location. The location ofthe boundary area between the settled bed and the settling slurry isdetermined preferably either by pressure difference measurements or bymeans of a radio-frequency level controller.

Yet another embodiment of this invention resides in an apparatus forcarrying out the process described above. This apparatus comprises anessentially cylindrical housing confined by a wall. Preferably thishousing will have a circular cross-section. At least one filter portionis arranged within said wall; the filter portion will be of thecharacteristic described above in connection with the process. Afiltrate receiver chamber is associated with each filter portion. In thepreferred embodiment the housing is provided with one filter portion andone filtrate receiver chamber. If a plurality of filter portions areused along the housing it is preferred to have a filtrate receiverchamber individually associated with each filter portion. The apparatusof this invention is further provided with an inlet conduit for a washfluid which is provided with a wash fluid outlet. For each axiallydisplaced filter portion a separate wash fluid outlet is provided for.The wash fluid outlet is arranged within said housing so that the washfluid flowing toward the respective filter will flow under the angledescribed above with respect to the main flow direction of the polymerslurry. A polymer slurry removal conduit is associated with the lowerend of the housing. Preferably in this slurry removal conduit a controlvalve is arranged with allows the controlled withdrawal of the slurry.

For safe operation of the apparatus it is preferred that there is a washfluid pressure controller associated with the inlet conduit for the washfluid as well as a filtrate pressure controller associated with thefiltrate receiver chamber, such as to insure a constant pressuredifferential between the wash fluid outlet and the filter.

The present invention is applicable to a wide variety of polymerslurries. Slurries of polyolefins and poly(arylene sulfides) arepresently preferred. The particle size of the solid polymer particles inthe slurry is not particularly critical. However, the invention is mostdesirable in cases where the polymer slurries involve very small polymerparticles, such as slurries wherein more than 50 weight percent of thetotal solid polymer has a particle size of 100 microns or less.

As pointed out above, it is presently preferred that the process of thisinvention involves both a cross extraction and a settling of the polymerslurry. This can also be expressed in fluid content of the slurry. Sinceall the fluids injected by means of the slurry and by means of the washfluid must equal the total quantity of fluids withdrawn from thefiltrate receiver chamber and with the produced settled bed, thepreferred embodiment of this invention resides in a process where thetotal quantity of fluids withdrawn via the purified polymer slurry issubstantially less than the total quantity of fluids contained in theslurry injected into the housing. More specifically yet, the settlingconcentration of the solid polymer particles in the slurry is such thatthe product slurry i.e. the settled bed or concentrated slurrypreferably contains at least about 1.3 times, and preferably at least1.5 times the quantity of solid polymer particles in the feed slurryvolume.

The column will be operated under balanced positive pressures which areregulated with pumps, flow control valves, pressure regulators, sensingdevices, etc., which are known in the art. For reasons of safety,economy and convenience the maximum pressure generally will not exceedabout 100 psia (0.69 MPa) and can preferably range from about 20 toabout 75 psia (0.14-0.52 MPa). The pressures employed in the variouslocations of the column will vary as to be described in more detaillater.

The column can be operated over a reasonable span of temperaturesproviding essentially liquid phase conditions exist which will dependupon the nature of the slurry components and wash liquids employed. Withthe polypropylene reactor effluent previously mentioned, for example, atemperature ranging from about 20° to about 150° C. can be used. With aPPS-water slurry the temperature can range from about 20° to about 200°C.

The nature of wash liquids employed will depend upon the type of slurryto be treated and will generally be conventional in nature, e.g. water,hydrocarbons such as n-hexane, propylene, etc., ketones such as acetone,alcohols such as methanol and epoxy compounds such as propylene oxide.Preferably the wash fluid is the same liquid that is present as theliquid phase of the slurry but is essentially free of the product to beremoved, such as catalyst residues or liquid polymers or amorphouspolymers.

Thus, for example, the polypropylene reactor effluent and thePPS-salt-water slurry can be treated in the apparatus of the inventionto wash out soluble components through an annular filter or filterslocated approximately in the center of the column. Excess slurry liquidpasses through the bed and out the filter leaving particles trapped atthe interface. The formation of the dense phase takes place at aninterface area above the filter and above the wash outlet; from thisinterface area down the flow of the solid/fluid mixture can becharacterized as plug flow. The purified, concentrated slurry ispreferably intermittently withdrawn in pulses from the column bottom toavoid upsetting the vertical pressure differential. The total volume ofslurry and wash liquid admitted to the column will be sufficient tomaintain the height of settled bed at a generally constant level abovethe wash inlet.

The feed slurry moves axially along the column; the liquid flows throughthe interface between the slurry and the settled bed, and flows throughthe settled bed. The wash fluid passes from the wash fluid outlet (whichis the inlet for the wash fluid into the column) to the filter andthereby displaces the slurry fluid (sometimes called mother liquor),which leaves the column through the filter. The volume of wash fluidinjected is preferably sufficient to replace all of the slurry fluid.The volume flow rate of the wash fluid is thus equal to or larger thanthe volume flow rate of the liquid filled space between the polymerparticle in the settled bed.

In the embodiments of the invention shown in FIGS. 1 and 2, a column 1is employed containing an annular filter zone 2 for washing of theslurry with suitable instrumentation to permit automatic operation. Thecolumn can possess any height to diameter ratio which permits thedesired washing and settling features to occur. Generally, the ratio canrange from about 3.5:1 to about 10:1. Ratios in the range of about 4:1to about 5:1 are presently preferred for reasons of economy as well asefficiency.

The column size will be dependent upon the production rate desired. Forexample, for about a 20 lbs/hour (9.1 kg/hr) production rate, a columnof about 18 inches high and 4 inches in diameter 45.7 cm×10.2 cm) havinga height to diameter ratio of about 4.5:1 can be employed. For aproduction rate of about 9000 lbs/hour (4100 kg/hr), a column about 18feet high and 4 feet in diameter (5.5×1.2 m) can be employed.

One or more annular filter zones 2 can be employed with the column 1.Generally, one zone is preferred to simplify the flow pattern of thewash liquid and to avoid the problem of wash effluent reentering thecolumn that can occur with multiple filter zones and upsets in thepressure.

The length of the filter zone can vary from about 1/4 the columndiameter to about 1.5 times the column diameter depending to some extentupon the nature of the slurry employed. It is found that a length aboutequal to one column diameter gives acceptable results.

The location of the filter zone is dependent to some extent upon thenature of the slurry employed and the sensitivity of the control devicesused in regulating the height of the slurry in the column. Acceptableresults can be obtained with a filter zone positioned such that its topterminates from about 0.5 to about 1 times the column diameter below themaximum height of the settled bed in the column.

The tube through which the wash liquid enters the column is sized toprovide the quantity of liquid required. For example, a tube 1 inch (2.5cm) in diameter can be empoyed with a 4 inch (10.2 cm) diameter columnand a tube 12 inches (30.5 cm) in diameter can be employed with a column4 feet (1.2 m) in diameter. The tube enters the column at a convenientlocation above the uppermost level of the contained liquid to avoiddisrupting plug flow of the slurry, extends downwardly in the columncenter and can terminate near the top area of the filter zone or at someintermediate location near the bottom of the filter zone. In the latterinstance, the tube bottom can be sealed and the wash liquid expelledradially outward through porous walls or the like in the vicinity of thefilter. In the first instance, the tube bottom is open and the walls canbe solid or porous for several tube diameters from its lower end.

The quantity of wash liquid employed will be equivalent in volume to theamount of filtrate exiting the filter zone and of course is dependentupon column size and nature of the slurry preferably the volume of washliquid will be equal to or slightly larger than the fluid of the slurryreplaced. The volume of filtrate is preferably equal to that of theliquid in the entering slurry. The wash liquid advantageously is atleast equal in volume to that in the product and preferably should be inslight excess. The original liquid in the entering slurry flows mostlyout of the top portion of the filter. The slight excess of wash liquid(if any) leaves mostly near bottom of filter. The wash liquid can besupplied manually or automatically to the column by using suitablemeasuring and supplying means known in the art such as pumps, flowcontrol devices, rotameters, etc.

The slurry enters the column at a convenient location 3 above theuppermost level 4 of the settled bed contained in the slurry filledcolumn. It is supplied on demand from a pressurized supply 5 when thesupply pressure exceeds that in the column after a pulse of washed,concentrated slurry passes from the column 1 via conduit 6. A pressurecontrol valve 25 controls the flow of slurry. When the pressure of theentering slurry in inlet 3 exceeds the pressure desired in column 1, apressure control valve 25 opens to allow the entering slurry to be atleast partially or completely returned to the slurry source by way ofconduit 24.

The control mechanism of this process and apparatus constitutes another,preferred embodiment of this invention. As shown in FIG. 1 the pressureof the column is detected near the bottom of the column at a location 8.The pressure is also measured in the column above and below the level 4of the settled bed, namely at locations 9 and 10. In the drawing "P1" isintended to indicate pressure display and/or pressure recorder units.The three pressure signals are converted into two pressure differencesignals in units 11 and 12 and these pressure difference signals aretransmitted to a relay 13. This relay 13 opens or closes the productslurry valve 14 in the product line 6.

The wash fluid is injected into the wash column via conduit 15. The washfluid is withdrawn from a fluid supply not shown via a flow controlleror rotameter 16. The wash fluid is passed through a pressure controlvalve 17, a backflow check valve 18 and a cartridge filter 19.

The locus 20 of wash fluid discharge is located approximately at theupper end of the annular filter 2. The wash fluid is discharged at apressure significantly above the pressure in the filtrate collectingchamber 21. This filtrate collecting chamber 21 surrounds the annularfilter 2. The wash fluid thus passes from the orifice 20 through theslurry and replaces the slurry fluid by the wash fluid. The slurry fluidleaves the column 1 through the filter 2 via a conduit 22 which isoperated under controlled pressure by means of a pressure control valve23.

The main difference in the apparatus shown in FIG. 2 and that shown inFIG. 1 resides in the means of detecting the level of the settled bed.In the apparatus of FIG. 2 a radio-frequency level controller 30 isshown which has an antenna 31 and which generates a signalrepresentative of the location of the saturated bed level 4 in thehousing. This signal together with a pressure difference signal are usedto operate the relay 13 which in turn actuates the product valve 14. Thepressure difference signal is representative of the pressure differencebetween the pressure in the bottom of the column at location 8 and thepressure in the top of the column at location 10.

EXAMPLE

A slurry consisting of 25 wt. % solids, namely salt (NaCl) filled PPS,and water was charged to a glass column 4 inches in diameter and 18inches in height as schematically shown in the drawing. The glass columnwas filled with the slurry and pressured and the water flow was adjustedby manually throttling valves and observing pressure indicators and aflow meter on the wash inlet. With a bed level established, and aneffluent flow established, the product valve was periodically opened toallow plug flow to lower the bed level in 0.5 inch increments. To drawproduct in this manner, it was necessary to have a pressure of at least30 psig at the bottom of the column. This minimum pressure is a functionof the characteristics of the slurry and the ratio of valve size tocolumn diameter. This minimum pressure becomes the basis for setting theother pressures since it drops when the product valve is opened and mustrecover before product can be taken again. The recovery of pressure isnot instantaneous through a settled bed and may take 10 to 40 seconds.The pressure on the effluent side of the filter must be at least as highas the product valve pressure and the top of the column pressure must behigh enough above the effluent pressure to provide flow through theporous settled bed and the filter. It is then necessary to set the washinlet pressure slightly above the slurry inlet pressure and adjust washflow rate to an appropriate ratio with effluent flow rate. With theaforementioned 25 wt. % solids in water and the solids being about 50wt. % salt, product takeoff consisted of a solid saturated slurry ofabout 50 wt. % solids, the solids containing 98 weight % polymer and 2weight % salt. The process of this invention thus has reduced the saltcontent of the PPS slurry significantly and has resulted in a productwith an acceptable salt content.

Reasonable variations and modifications which will become apparent tothose skilled in the art can be made in this invention without departingfrom the spirit and scope thereof.

What is claimed is:
 1. Apparatus for cross-extracting a polymer slurrycomprising(a) an essentially cylindrical housing confined by a wall, (b)at least one filter portion in said wall, (c) a filtrate receiverchamber associated with each filter portion, (d) an inlet conduit with awash fluid outlet, said wash fluid outlet being located inside of saidhousing so that wash fluid injected will flow under a substantial anglewith respect to the longitudinal axis of said housing to said filterportion, (e) a slurry removal conduit associated with the lower end ofsaid housing, and (f) a slurry inlet conduit connecting the upperportion of said housing with a source of a polymer slurry.
 2. Apparatusin accordance with claim 1 comprising a filtrate pressure controllerassociated with said filtrate receiver chamber and a wash fluid pressurecontroller associated with said wash fluid inlet conduit such as toestablish a constant pressure differential between said wash fluidoutlet and said filtrate receiver chamber.
 3. Apparatus in accordancewith claim 1 comprising pressure sensing means associated with thesection of the housing downstream of the lowest filter portion, levelsensing means to determine the location of the level between a settledbed of the slurry and a settling slurry and comprising controller meanswhich open a valve in said slurry removal conduit responsive to thesignals generated by said pressure sensing means and said level sensingmeans.
 4. Apparatus in accordance with claim 3 wherein said levelsensing means comprise a radio-frequency level controller.
 5. Apparatusin accordance with claim 1 wherein said cylindrical housing has anessentially circular cross-section and wherein each filter portion is anannular filter portion and wherein each filtrate receiver chamber is anannular chamber surrounding said filter portion.
 6. Apparatus inaccordance with claim 2 comprising pressure sensing means associatedwith the section of the housing downstream of the lowest filter portion,level sensing means to determine the location of the level between asettled bed of the slurry and a settling slurry and comprisingcontroller means which open a valve in said slurry removal conduitresponsive to the signals generated by said pressure sensing means andsaid level sensing means.
 7. Apparatus in accordance with claim 1further comprising(g) a pressure controlled recycle loop for recyclingsaid slurry from said slurry inlet conduit to said source of slurry inresponse to a pressure signal representative of the pressure in saidslurry inlet conduit.
 8. Apparatus for cross-extracting a polymer slurrycomprising(a) an essentially circular cylindrical housing confined by awall, (b) at least one filter portion in said wall, the length of saidfilter portion being from about 0.25 to about 1.5 times the diameter ofsaid circular cylindrical housing, (c) a filtrate receiver chamberassociated with said filter portion, (d) an inlet conduit with a washfluid outlet, said wash fluid outlet being located inside of saidhousing so that wash fluid injected will flow under a substantial angleof 40° to 90° with respect to the longitudinal axis of said housing tosaid filter portion, (e) a slurry removal conduit associated with thelower end of said housing, and (f) a slurry inlet conduit connecting theupper portion of said housing with a source of a polymer slurry, withthe further proviso that the top of said filter portion is located fromabout 0.5 to about 1 times the diameter of said circular cylindricalhousing below the maximum height of a settled bed of said slurry.